This invention relates to television camera systems in which a video signal is developed from one or more image pickup tubes and, in particular, to suppression circuits for eliminating from the video signal developed by the image pickup tubes the effect of illumination highlights present in the scene being viewed.
Modern day television cameras include one or more image pickup tubes for developing a video signal from the scene being viewed. The image pickup tube generally comprises an electron gun assembly for generating an electron beam which is used to scan a photoconductive target on which the light from the scene is imaged. Beam deflection coils placed around the pickup tube and driven by horizontal and vertical rate signal waveforms cause the electron beam to scan the photoconductive target in an orderly fashion to develop a video signal representative of the scene by virtue of the electron beam recharging the photoconductive target to cathode potential. The number of image tubes utilized in a television camera generally relates to the form of video signal desired, i.e., a single pickup tube may suffice for a monochrome video signal, while a color video signal generally requires two or more tubes for developing the complete color signal. A typical color camera utlizing image pickup tubes of the above-described form is the RCA TK-76 camera manufactured by the RCA Corporation, Camden, New Jersey, U.S.A. In the TK-76, the incoming light from the scene is optically split into its red, green, and blue components, and each portion is imaged onto the target of individual image pickup tubes for developing the red, green, and blue color portions of a video signal representative of the scene being viewed. The color signals from the image pickup tubes are then processed in known circuitry to form the composite video signal, for example, to NTSC standards.
Image pickup tubes utilizing a photoconductive target on which the light from a scene is imaged, as described above, may develop levels of target voltage modulation, upon exposure to above normal illumination highlights in the scene, that cannot be recharged fully by the normal steady state beam current which is used to develop the output video signal from the image pickup tube by recharging the photoconductive target. Where the highlight exposure is relatively intense, for example, as illustrated by a point source of light in the scene, many passes or scans of the target by the beam will be required for complete recharge of the target. If the highlight in question is moving, an effect known as comet-tailing results in which the moving highlight is trailed by a tail of light. In a color camera system, the comet-tailing is particularly troublesome since the comet tails may flare up as spurious bright colors in the final display not related to the highlight or scene background.